U.S. Pat. No. 4,357,827 to McConnell describes an apparatus for determining the weight of liquid flowing to or from a test site. The McConnell apparatus comprises a vessel 12 for containing a liquid to be used in testing the performance of a sample 30. Vessel 12, which is supported by a balance 16, includes a siphon tube 22 which is connected to the bottom of a test cell 26 by tubing 24. Test cell 26, which has an upper surface 28 on which the test specimen is placed, is mounted on an O-shaped support 32. As exemplified in FIGS. 1 and 2 of U.S. Pat. No. 4,357,827, test cell 26 is a flat disk-shaped plate having a hole 56 in the center through which liquid can flow to the test specimen via tubing 24. Each of the test cells 26, 60 and 66 illustrated in U.S. Pat. No. 4,357,827 is provided with a flange 58 so that the cell will fit into the aforementioned ring-shaped support 32.